The present invention relates to the structure of a semiconductor laser device which oscillates in the fundamental mode at a high output.
As shown in FIG. 12 by way of example, a priorart semiconductor laser has guided laser radiation by establishing the difference of effective refractive indices inside and outside a laser stripe in such a way that a region 6 exhibitive of an absorptivity for the laser radiation is disposed at a position spaced several hundred nm from the active layer 3 of the semiconductor laser. In case of the laser having such a structure, in a region of intense laser radiation, carriers within the active layer are dissipated by the stimulated emission of the laser radiation, and hence, the refractive index of the active layer increases. Especially the index increase at the part of intense radiation within the active layer becomes greater than at the other parts. Therefore, the self-focusing in which the radiation concentrates on the particular part more and more takes place to lead to the self-filamentation. Under the condition of the self-filamentation, the laser beam of the semiconductor laser shifts with the movement of a filament until the oscillation thereof shifts to a mode of higher order. For the purpose of preventing this drawback and attaining the fundamental mode oscillation, the width of the stripe needs to be reduced. When the stripe width is made smaller, the laser beam cannot shift, and the higher-order mode oscillation is cut off. With the narrow stripe, however, the light density of the laser heightens, and unfavorably the crystal thereof becomes liable to optical damages. It can accordingly be said that the reduction of the stripe width is demeritorious for the fabrication of a high-output laser.
The literature of the prior art forming the background of the present invention is, for example, "Applied Physics Letters," Vol. 30, No. 12, 1977, page 649.